The present invention relates generally to gasoline internal combustion engines and more particularly concerns a method and apparatus for mixing and modulating liquid fuel and intake air in order to reduce the undesirable exhaust emissions from such engines.
In nearly all gasoline engines used in automotive applications today, the fuel and air are metered and mixed by a carburetor connected to the intake manifold of the engine. While these carburetors differ considerably in detail, their overall operation is basically the same in that fuel is drawn from a float-controlled fuel reservoir through one or more small fuel jets by the pressure drop created as the air flows through a fixed venturi section formed in the throat of the carburetor. During normal operation the air flow through the carburetor and, hence, the amount of fuel drawn through the metering jets is controlled by a butterfly-valve-type throttle plate. However, because the air flow through the carburetor varies markedly during different engine operating conditions, such as: idle, acceleration, full throttle, and deceleration, conventional carburetors are commonly provided with separate idle jets, acceleration pumps, and multiple venturi sections. Even so, the metering function of the carburetor falls short of providing the desired air-fuel mixture to the engine at all operating conditions and the mixing function performed by the carburetor is even worse.
Except at idle essentially all of the mixing in a conventional carburetor occurs as the fuel and air pass together through the throttle opening. Assuming atmospheric pressure of 29.9 inches of mercury (in. Hg.) exists at the carburetor inlet, the air flow through the throttle opening will be at sonic velocity when the pressure at the throttle opening is 53% of atmospheric. This is equal to a pressure of 15.6 in. Hg. and is referred to as the critical pressure. However, since it is common to measure the condition within the intake manifold in terms of inches of mercury vacuum rather than pressure, this critical pressure is equal to 14.3 in. Hg. vacuum (29.9 - 15.6 = 14.3) and this condition will be hereinafter referred to as the threshold vacuum. Moreover, due to the shapes of the carburetor throat and throttle plate, a vacuum in the intake manifold only slightly below threshold vacuum will just produce sonic velocity through the throttle opening. This condition, which is referred to hereinafter as the "unchoking point", occurs at about 12 in. Hg. vacuum for a typical carburetor or 17.9 in. Hg. pressure (29.9 - 12 = 17.9). Sonic velocity of the intake air through the throttle opening also occurs at manifold vacuums above the unchoking point, in other words, in the range of about 12 to 24 in. Hg. during normal operation. Expressed slightly different, when the pressure in the intake manifold of a typical carburetor is about 60% of the pressure at the carburetor inlet (60% .times. 29.9 = 17.9) or less, sonic velocity of the intake air occurs through the throttle opening. For reasons explained below, the present invention provides sonic velocity over a wider range and even when the pressure in the intake manifold is substantially more than 60% of the pressure at the inlet.
When the velocity of the intake air through the throttle opening is at sonic velocity, the high velocity air divides the liquid fuel into fine droplets. However, because the throttle plate slopes across the carburetor throat below the fuel jet, nearly all of the fuel and about half of the air flows through the lower throttle opening but only a small amount of fuel passes with the other half of the air through the upper throttle opening. Although some mixing of these two streams of fuel and air does occur below the throttle plate, the asymmetrical distribution of the fuel in the intake air is substantially never completely overcome.
At manifold vacuum conditions below the unchoking point, the mixing of fuel and air by the carburetor is even worse. This normally occurs at all manifold vacuum conditions below about 12 inches Hg. when the engine is accelerated or under load. Under these conditions, the air flow is below sonic velocity, frequently well below, and more fuel is being introduced. The fuel distribution is still asymmetric and mixing at the throttle opening and below is even less effective due to the much larger droplets which are formed by the lower velocity air. In addition, if the carburetor includes an accelerator pump, as most do, the additional squirt of fuel that it provides usually comes just when the throttle is being opened rapidly and the air velocity is falling well below sonic. Thus, a stream of liquid fuel may pass directly into the intake manifold.
During idle conditions, the fuel is typically introduced through an idle jet located just below the lower side of the throttle plate when it is in the idle position. Naturally, this results is asymmetrical fuel distribution in the intake air and although the air flow through the throttle opening is typically at sonic velocity during idling conditions, the idle fuel is not very effectively or uniformly mixed with the intake air.
Largely, as a result of these shortcomings in current carburetor arrangements, there are wide cylinder to cylinder and cycle to cycle variations in the ratio and amount of fuel and air delivered to the engine at different operating conditions. This is true even when the metering function of the carburetor initially provides the desired air-fuel ratio at the manifold inlet because the mixing function of the carburetor is so poorly performed that streams of liquid fuel frequently pass into the intake manifold, wetting portions of the manifold walls and actually collecting in pools of liquid fuel in certain areas of the manifold, and some of this unmixed liquid fuel is inducted into the engine cylinders.
In an effort to overcome this situation, various arrangements have been adopted to heat the intake manifold in order to vaporize the liquid fuel prior to induction into the engine cylinders. The most common of such arrangements are hot spots and heat risers from the exhaust manifold to heat the area of the intake manifold immediately below the carburetor. A hot water path through the intake manifold is also frequently employed. Even with these arrangements, however, a completely uniform air-fuel mixture throughout the manifold is rarely achieved. Consequently, the air-fuel mixture delivered to some of the cylinders is often too rich to achieve complete combustion. On the other hand, the air-fuel mixture delivered to other cylinders is at times too lean to achieve proper burning and this causes those cylinders to misfire. As used in the present application, it will be understood that a rich air-fuel mixture is one that contains more than one pound of fuel for every 15.5 pounds of air and that a lean air-fuel mixture is one that contains less than one pound of fuel for every 15.5 pounds of air.
Whether the problem is misfiring due to too lean an air-fuel mixture or incomplete combustion due to too rich a mixture, the result is that unburned fuel is exhausted from the cylinders. This is undesirable not only because of the loss in power and efficiency that results but also because these unburned or incompletely burned fuel components pass into the atmosphere as undesirable pollutants.
The principal air pollutants emanating from internal combustion engines have been identified as unburned hydrocarbons (HC), carbon monoxide (CO), and the oxides of nitrogen (NO.sub.x). The desired end products of complete combustion of the fuel and air, of course, would be carbon dioxide and water with only a trace of other constituents in the presence of unreacted nitrogen.
Prior to enactment of federal and state standards on exhaust emissions, a standard automobile engine in good running condition would produce an average of about 900 ppm HC, 3.9% CO and 1075 ppm NO.sub.x during normal operation. The initial federal standards, effective January, 1968, covered only HC and CO emissions and were stated in terms of concentrations of 275 ppm HC and 1.5% CO. In terms of the subsequently prescribed seven-mode cycle test which is to simulate a typical 20 minute trip of a car from cold start through city traffic, the 1968 federal standards correspond to about 3.4 g/mi HC and 34 g/mi CO. Effective January 1970, these were reduced to 2.2 g/mi HC and 23 g/mi CO which correspond to concentrations of about 180 ppm HC and 1% CO for the average car.
The standards originally proposed for 1975 (Fed. Reg. Vol. 33, No. 108, June 4, 1968) were 0.5 g/mi (about 40 ppm) of hydrocarbon, 11.0 g/mi (about 0.5%) of CO, and 0.9 g/mi (about 240 ppm) of NO.sub.x, based on the 7-mode cycle, that was adopted. In 1970, new standards for 1975 and 1976 were established along with a new driving cycle (Fed. Reg. Vol. 35, No. 219, Nov. 10, 1970). On 1975 model cars, hydrocarbon must not exceed 0.46 g/mi (about 37 ppm) and CO 4.7 g/mi (about 0.2%). On 1976 model cars, it is proposed that NO.sub.x be limited to 0.4 g/mi (about 110 ppm). These emissions are to be obtained using a constant volume sampling system and while driving a car through a new 22 minute driving cycle. It will be appreciated that the standards were hence reduced in two ways, by lowering the actual numbers and also by changing the analytical method.
The automobile engine manufacturers were able--with some difficulties--to meet the 1968 federal emission standards primarily by adopting one or more of the following engine modifications:
1. retarding the spark-ignition PA1 2. recalibrating the carburetor for leaner air-fuel mixtures PA1 3. heating the intake manifold PA1 4. changing valve timing PA1 5. increasing stroke to bore ratio PA1 6. injecting air into the exhaust manifold PA1 7. improving combustion chamber design
Further improvements in these areas have also made it possible to meet the federal standards for 1970.
However, the stringent nature of the federal exhaust emission standards for 1975 are such that it is believed that even the most effective combination of all of the above measures will not be sufficient even with added catalytic or thermal reactors and, indeed, serious concern is being voiced as to whether or not the internal combustion engine can economically be made sufficiently polution free to meet these projected standards.